The present invention relates generally to a method for improving the quality of printing processes involving pixel depletion, and pertains more particularly to a method for adaptively selecting pixel depletion mask patterns based on the quality of individual printing elements in a swath printer.
In order to print an image such as a text document, a graphic or a photograph, on a print medium such as paper or transparency material, a typical high quality color inkjet printer prints a band, or xe2x80x9cswathxe2x80x9d, at a time of colored ink drops which correspond to the data pixels that comprise the image. Typically, four different color inks (cyan, magenta, yellow, and black) are used by the printer to print the range of colors contained in the image. By printing successive swaths, the document or image is completely formed on the print medium. Such inkjet printers are described by W. J. Lloyd and H. T. Taub in xe2x80x9cInk Jet Devices,xe2x80x9d Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988). The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994)], incorporated herein by reference.
When printing a swath, certain high-density regions of the printed medium may receive a relatively large total amount of ink because of the darkness of the image in those regions. In these regions, the ink may not dry quickly enough to avoid blotting onto the preceding or following page in the output tray of the printer, or smearing when handled by the user. Large amounts of ink in an area can cause certain print media to buckle or cockle, or wick into adjacent lower-density regions of the medium, thus degrading the image. In addition, ink may be wasted in high-density regions, because ink drops typically spread out and overlap each other on the print medium, and consequently a lesser total amount of ink in these regions may produce an equal or better quality print of the image.
As a result, in order to reduce ink drying time, paper cockling, and wicking, and/or to decrease ink usage, xe2x80x9cpixel depletionxe2x80x9d processes (to reduce the density of high-density image regions) have been employed in thermal inkjet printers. Typically, depletion masks have been used to identify pixels targeted for depletion. See, e.g., U.S. Pat. No. 5,706,414 to Pritchard, assigned to the assignee of the present invention and incorporated herein by reference in its entirety. The depletion masks are typically chosen so as to reduce the density enough to conserve ink and prevent defects (such as those described above) which are due to excessive inking of the medium, but not so as to be visually perceptible to the user as xe2x80x9cholesxe2x80x9d or unprinted spaces in the printed image. Beneficial depletion masks, therefore, carefully control the quantity and location of the depleted pixels within and between swaths.
Unfortunately, the printhead cartridges containing the individual printing elements which controllably deposit the ink drops on the print medium may degrade during the course of their useful life, resulting in misdirected ink drops which are not deposited in the intended location, or no ink drops at all. Each defective printing element typically results in a row or line of unprinted spaces on the print medium. In addition, depending on the location of these degraded printing elements in the printhead cartridge and the locations of depleted pixels in the depletion mask, more total unprinted spaces, or unprinted spaces clustered together in a single region can occur, resulting in a visually unpleasing printed medium with poor image quality. Accordingly, it would be highly desirable to have a way to mitigate the adverse impact on print image quality of defective printing elements in the printer.
In a preferred embodiment, the present invention provides a new and improved swath printing system that adaptively depletes pixels based on the health of the printing elements so as to achieve a high level of image quality in the printed output despite the presence in the printing system of the defective printing elements. The invention is scalable such that it can be cost-effectively embodied in both high-end and low-end printing systems to mitigate the adverse effects of the defective printing elements. An image processor receives image data, and processes it for printing according to a depletion mask which depletes a predetermined percentage of the pixels in the image data. Following processing, the image processor issues control commands to a print mechanism which emits in response drops of colored ink onto specified locations of a print medium to print the image. The depletion mask defines the location of the individual pixel positions to be depleted, and selects those locations such that relatively more of them positions correspond to pixel locations that will be printed by defective printing elements, and relatively fewer of them corresponding to pixel locations for functional printing elements. A printing element quality detector identifies the defective printing elements, allowing the image processor to specify where in the depletion mask the depleting positions should be located, or select an appropriate depletion mask from a group of alternate ones.
The present invention may also be implemented as a method for printing pixel rows of an image swath with a swath printer. Defective printing elements of the swath printer are identified, and one or more of the defective printing elements are mapped to the corresponding pixel rows that will be printed by these defective elements. A desired percentage of pixel depletion for the swath is achieved by disabling printing of at least some individual pixel positions in at least one of the pixel rows corresponding to defective printing elements. Printing is preferably disabled for more of the individual pixel positions in pixel rows corresponding to defective printing elements than for pixel rows corresponding to functional printing elements; this may include disabling printing for all of the individual pixel positions in pixel rows corresponding to defective printing elements before disabling printing for any positions in pixel rows corresponding to functional printing elements. In some embodiments, the method modifies image data by replacing the image data pixels corresponding to the disabled pixel positions with non-printing pixels. In other embodiments, the method segregates from the image data a color plane of pixel data for a predetermined ink color, and modifying the color plane pixel data by disabling printing of the color plane pixels corresponding to the disabled pixel positions
The present invention may additionally be implemented as a method for printing rows and columns of an image swath with a swath printer, preferably in a single pass. Defective printing elements of the swath printer are identified, and a depletion mask having a predetermined percentage of depleted pixels is provided. The depletion mask has relatively more depleted pixel positions in rows corresponding to the defective printing elements and relatively fewer depleted pixel positions in rows corresponding to functional printing elements. The image swath is processed using the depletion mask in order to deplete the predetermined percentage of pixels from the swath, by disabling printing of pixel positions in the swath at the depleted pixel positions specified by the depletion mask. The depletion mask may be constructed based on the location of the defective printing elements; or alternatively one of a predefined set of depletion mask primitives may be selected based on the location of the defective printing elements, and tiled to form the depletion mask. The depletion mask is preferably arranged such that no two adjacent pixel positions in a vertical column are depleted.
Another embodiment of the present invention is a program storage medium containing a program of instructions executable by a computing apparatus for preparing an image data file having rows and columns of image pixels for printing on a swath printer. Different segments of the instructions are configured to receive data identifying defective printing elements of the swath printer; map at least some of the defective printing elements to corresponding lower print quality rows of the image data file; specify a depletion mask having a predetermined percentage of depleted pixels with more depleted pixel positions in lower print quality rows and fewer depleted pixel positions in other rows; and modify the image data file by replacing image data pixels corresponding to depleted pixel positions with non-printing data pixels.